> making something new from the ground up doesn't seem like a good idea.
It isn't actually terribly difficult, the most difficult part being driver support. By using Linux as a base, Google gained a lot of driver support by default, among other things like a well-known base environment.
> explain if a micro kernel makes his job easier or harder
Well, a microkernel usually has well-defined interfaces that allow drivers to communicate with the kernel, instead of these interfaces being decided by the compiler at compile-time. This in turn offers the guarantees of "ABI/API stability" that the Linux kernel usually offers to userspace programs: drivers don't need to be recompiled to work with a new kernel, so you can actually have binary drivers that continue to "work" long after the vendor stopped working on them. This also offers more room for stability guarantees (a driver that crashes can be restarted, as it's not the kernel panicking), and security (drivers don't have access to each other's data).
On the other hand, you trade a bit of performance and flexibility (from the developer's perspective) for this, by committing not to break compatibility. By now, advantages and inconvenient of microkernels vs monolithic ones are well understood and documented.
What's in for Google is mostly licensing-related: since they're making it; they are free to pick their favourite license. And I am pretty sure Google has been uncomfortable with the GPL for a while now. The same could be said about its hardware partners, and manufacturers that have to provide the source code for their drivers (when they comply at all).
Since a GPL kernel is one of the best things that happened to Android so far (allowing custom roms, open source projects, more transparency), I am extremely weary of fuschia, and looking forward to postmarketos as an alternative to android.